Near Field Communication (NFC) enables contactless short range communication between two devices, typically requiring distance of 4 cm or less to initiate a connection. The connection can be done much faster than other communication technologies like Bluetooth or WiFi. The user only needs to bring two NFC supported devices closer and data will be transferred automatically. Some of its applications include credit card payment, ticketing, content sharing, quick pairing, and etc.
In general, an NFC device requires very low power or no power for transmission when it operates in tag or card emulation mode. An NFC tag/card device will first detect a radio frequency (RF) magnetic field from NFC reader device. The magnetic field energy is used to power the load component in the NFC tag/card device and transfer the data using passive load modulation (PLM). The term “passive” is used here because there is no need for the NFC tag/card device to use its own energy for the transmission. Thus, when implemented in any mobile phone or tablet, it still works when the battery dies and it does not affect device battery consumption.
While having good advantages, passive load modulation has its drawbacks. The amount of magnetic field that can be absorbed by the NFC tag/card device is limited to the antenna coupling between two NFC devices. Antenna coupling performance can deteriorate, for example, when a smaller antenna is used or when two NFC devices are further apart.
Another NFC tag/card emulation technique is Active Load Modulation (ALM). This technique is getting popular as there is a strong interest in integrating NFC technology into mobile phones or tablets. Mobile phones or tablets have their own battery power, and thus, the NFC tag/card devices will be able to utilize their own battery power to generate the magnetic field during load modulation. ALM mode is also more suitable for smaller antenna implementation in the device and has better performance than the PLM mode. However, for both PLM and ALM modes, normally clock recovery is required to ensure synchronous transmission between NFC devices.
A known NFC using analog phase-locked loop (PLL) to control ALM is disclosed in U.S. Pat. No. 8,934,836 B2. FIG. 1 shows the circuitry design of the NFC between two devices disclosed in U.S. Pat. No. 8,934,836 B2.
The NFC Device 2 includes a clock recovery process 200 in the Analog module to recover the clock fR1 from NFC Device 1 magnetic field. The PLL is configured to receive one of the recovered clock and reference clock, and to utilize the received clock to control the active load modulation at both the digital receiver module and digital transmitter module. A driver is also used to adjust the amplitude of the voltage across the antenna. Since the clock recovery process is able to recover the clock fR1 from NFC Device 1 magnetic field, the transmission signal clock from NFC Device 2 can be exactly the same as fR1. As a result, the NFC Device 1 will receive active load modulated signal from NFC Device 2 as conventional tag passive load modulated signal.
Unfortunately, U.S. Pat. No. 8,934,836 B2 requires a complex analog RF front end with the clock recovery process 200 to recover the clock fR1, and subsequently to use fR1 to control both the ALM reception and transmission. Further, the clock recovery process 200 and magnetic field generation may require the analog PLL forced in open loop state, which is difficult to implement in practice. Simpler or less complex of RF analog RF front end is desired for combo-chipset implementation where the size of analog RF front end could be limited.
In light of the above, those skilled in the art are striving to improve the clock recovery process for current NFC.